The present invention relates to an antistatic resin composition. More particularly, it relates to an antistatic resin composition capable of giving antistatic films having a high degree of transparency.
Because of their excellent chemical and physical properties, thermoplastic resins such as polyolefin, polystyrene and polyvinyl chloride resins have been widely used in various shaped articles inclusive of films but since they possess high insulating properties as do many other plastics, they are ready to be statically charged on friction, for instance, so that the film surface is easily soiled with attracted dust and dirt. Furthermore, depending on the place of use, sparking due to discharge of accumulated static electricity may cause various troubles such as ignition of inflammable solvents and destruction of the internal circuits of integrated circuit elements. To avoid these and other troubles, many attempts have been made heretofore to impart antistaticity to the packaging films, bags, boxes and other products made of those resins.
Among the hitherto-known methods for imparting antistaticity to a thermoplastic resin are:
(1) The surface of a shaped article is coated with a variety of surface active agents. PA1 (2) A variety of surface active agents are internally added. PA1 (3) Electrically conductive fillers such as carbon black, carbon fiber, metal powder, metal fiber, etc. are internally added. PA1 (4) A thin metallic film is formed on the surface of a shaped article.
However, the surface resistivity values of the antistatic plastic materials that can be manufactured by the surface or internal application of surfactants are generally more than 10.sup.10 ohms and such materials have been utilized only in applications not calling for a high degree of antistaticity.
Moreover, the shaped articles manufactured from an antistatic resin material prepared by the internal addition of an electrically conductive filler material are low in both surface resistivity and volume resistivity and although they are highly static-proof, even the film, for instance, is opaque and taking a bag as an example, the contents in the bag cannot be ascertained from the outside.
Of the antistatic materials carrying a thin metallic surface film, the transparent films on the market today are such that a noble metal is vapor-deposited on the surface. Though the vapor-deposited surface of such film is highly resistant to a build-up of static electricity, the most serious defect of this type of film is its high price and accordingly its use has been limited to special applications.
Thus, each of these known antistatic resin materials has its own advantages but also disadvantages. Under the circumstances, basically relying on the above-mentioned principle of internal addition of surfactants, the present inventors previously proposed a resin composition possessing improved antistaticity in Japanese Unexamined Patent Publication (Kokai Tokkyo Koho) No. 60-44536. This composition is an antistatic thermoplastic resin composition containing a defined type of organic matter and a metal salt such as lithium nitrate added to a thermoplastic resin and when the level of addition of the antistatic agent is approximately 0.5% by weight, a surface resistivity of about 10.sup.7 ohms could be accomplished. Moreover, the film had clarity and heat sealability as well.
However, the above antistatic thermoplastic resin composition prepared by adding a defined type of organic matter, namely a surfactant, and a metal salt as an antistatic agent to a thermoplastic resin is, indeed, satisfactory in antistatically and heat sealability, but since some of the metal salt used as a component of the antistatic agent is present in the form of relatively coarse grains in the stage of production, a part of them are trapped by the screen used for removal of foreign matter at the extruding machine so that the flow of the resin is hindered. As a consequence, the screen has to be replaced at short intervals and this detracts considerably from the efficiency of production. Furthermore, since a varying proportion of metal salt grains leak through the screen, it is difficult to ensure the uniformity of products. If the extrusion is carried out without use of such a screen, a large amount of metal salt grains enter into the resin and when a film is formed from such a resin composition, the metal salt grains are distributed on the film layer to seriously affect the film appearance. Moreover, since metal salt so added is not effectively utilized, the antistaticity of the film tends to be not as high as expected.
It is an object of the present invention to solve the above-mentioned problems associated with the presence of coarse metal salt particles in the antistatic thermoplastic resin composition and to obtain an thermoplastic resin composition having good antistaticity and appearance.
This and other objects of the invention will become apparent from the descriptions hereinafter.